The British Museum fought back yesterday in the increasingly bitter debate over its treatment of the Elgin Marbles, attacking the Greek authorities for allowing Parthenon carvings to "rot" on the Acropolis in Athens.

An international symposium on the cleaning of the marbles, organised by the British Museum as an exercise in scholarly bridge building, saw instead a startling escalation of the temperature of the debate.

Ian Jenkins, deputy keeper of Greek and Roman antiquities, admitted for the first time on behalf of the museum that the illegal scouring of the marbles in the 1930s, at the request of Lord Duveen, donor of a new gallery to house them, and the museum's attempts to cover this up, was a scandal.

But he said almost identical techniques, including wire brushing and scraping with metal chisels, had been used in Athens in the1950s on the Hephaesteum temple, and two of the finest carvings "still rot on the Parthenon as I speak".

Many in the Greek delegation were outraged at his remarks.

Ismini Trianti, director of the Acropolis Museum, said: "Rotting is a very hard word - that makes me very angry." She added: "The subject of this conference is not what we have done in Greece - we can organise another conference in Greece for this."

In the eyes of the Greek guests, the museum then compounded its crimes by serving a sandwich lunch for all the delegates within the Elgin gallery - particularly since they had been urged to touch the marbles for themselves to test how much damage had been done.

The charge d'affaires from the Greek embassy, Constantinos Bitsios, immediately complained to the museum staff. The museum has been villified recently for permitting evening fund raising parties in the gallery.

Mr Bitsios said: "This was not an official complaint from the embassy, but I remarked to them that this was disrespectful and tactless in the extreme in view of the recent controversy."

Most of the Greek delegates, and William St Clair, author of the books and recent papers on Lord Elgin and the marbles which triggered the conference, refused to eat.

Dr Jenkins protested that the marbles have been protected since their most recent 1970s cleaning with a waxy coat - which he said tended to highlight the effect of the 1930s work - and could not be damaged by being touched.

In his paper Dr Jenkins attacked Mr St Clair's interpretation of the facts, and said in one case he had mistaken a print from a damaged photographic negative for evidence of damage to a carving.

He insisted that the "overwhelming" cause of damage to the Parthenon sculptures was natural weathering in over 2,000 years on the Acropolis. The much debated patina, said to have been scraped off the marbles with bronze chisels by Lord Duveen's workmen in an attempt to restore a mythical pure classical whiteness, was not found on many of either the London carvings or those which had remained in Athens, and he believed it had been destroyed on most of the stones by weathering long before they ever came to Britain.

"The British Museum is not infallible, it is not the Pope. Its history has been a series of good intentions marred by the occasional cock-up, and the 1930s cleaning was such a cock-up."

Mr St Clair stuck to his guns, accusing the museum of "an institutionalised, systematic sustained and in many respects illegal attempt to withhold the facts". Far from being ancient history, he said the cover-up had lasted 60 years.

Alekkos Matzi, a member of the Greek team of experts, said the damage done to some of the carvings amounted to "a catastrophe". He got one of the few laughs of the day when he said that in the case of one horse's head, "the distortion is so extreme that it appears Roman".

A statement from the Greek ministry of culture last night said the cleaning of the temple of Hephaestus, in 1953, had been carried out by the American school of classical studies, and was carefully done and limited to removing salt crusts formed by rainwater.

In a lively general debate which followed the speeches, historian Sir John Boardman said, to audible gasps: "From an aesthetic point of view I'm absolutely delighted that all that orange and brown disappeared. One can see in old photographs how hideous they looked."